Hello. I just finished my conversion of my pre-facelift GS300 to an e-shift car and I wanted to give back the details since I got mixed information from this forum on my way. I don't know how to help you with sourcing the specifics parts. I had a 2001 parts car that I harvested the entire drivetrain from because I blew my original motor, and it's easiest to swap the engine and transmission as a unit. I also harvested the steering wheel, gauge cluster, connectors, terminals, and a number of other parts from the car. Honestly - just trawl a junkyard and you can probably find what you need except for the ECU.

To start off with, what you will need:
* 2001 GS300 ECU
* (optional, recommended) E-shift steering wheel
* (optional, recommended) 2001 GS300 / 97-00 GS400 clock spring (any e-shift car will have this clockspring)
* (optional, recommended) 2001+ gauge cluster
* (optional) 2001+ shifter fascia (you do not need the shifter assembly, just the part that says "M" instead of "4" in your shifter

The only difference that I've found in my car that is actually mandatory for e-shift is two switched-grounds and the 2001 ECU. It must be a 2001 specifically because that is the only year that e-shift was available on a pre-facelift GS. 2002+ cars have a completely different connector set for the ECU and are not compatible with pre-facelift harnesses.

The process for the ECU is as simple as removing the four bolts that enclose your ECU in the drivers-side front of the car and unplugging the six connectors, removing the ECU, and then transferring the two mounting brackets to your 2001 ECU. Remember - use JIS bits! There are two wires that you will need to add to the connector E5 - positions 4 and 14. The way e-shift works is that when in manual mode, 4 and 14 have battery voltage and trigger the shift when the wires are grounded. Grounding position 4 (SFTU) will upshift, and grounding position 14 (SFTD) will downshift. I recommend depinning another connector and adding the two terminals into your factory harness. These wires then have to be routed to the cabin for whatever controls. It does not need to be on the OEM steering wheel, but that is a plus; any way you prefer is acceptable as long as when you press the switch, the wire is shorted to ground. For my car, I routed it through the OEM grommets by punching a hole in it with a screwdriver and fishing them through with a solid copper wire. I used 20ga TXL wire from McMaster.

In the cabin, if you want to upgrade to an e-shift wheel, there is another concern: the clock spring. Non-eshift cars do not have two wires in the clock spring and the connector to support the buttons. This can be accessed by removing the steering wheel, airbag, and steering column covers. Disconnect the battery and wait 90 seconds, per TSRM, lest you blow your fancy new wheel. Remove your airbag using two T27 bolts on the left and right hidden under plastic covers, then remove the steering wheel center bolt and the wheel itself. Under the wheel are two JIS screws that need to come out (among other screws in the column). The clock spring is held in with a few small JIS screws to the column. Trace it to the C16 connector, which is a small black flat connector with four wires populated of six. You will need to add two terminals to the empty two positions; again, I harvested them from my parts car. Connect position 1 to SFTD, and position 2 to SFTU.

To display the gear like you would in an OEM car, all you need to do is remove the cluster and install one from any other e-shift GS300. Your odometer will be wrong; I have no fix for this right now. From what I have read on this forum, all 2nd generation GS clusters are completely interchangeable. I placed my 2001 cluster in my car and it was seamless, and also let me know that my replacement motor only had 130K on it (down from 250,061 on my original motor!) I did not have to do any wiring changes to keep the cluster functional, and it worked perfectly on both my 1998 and 2001 ECUs. It just doesn't show gear position on the 1998 ECU. Changing out the shifter bezel is as simple as removing the center console, unscrewing the shift ****, and the shifter fascia should be simply clipped on. Proceed with caution: old plastic will snap.

Here is what you DON'T need:
You do NOT need to change your shifter assembly, and in fact it will cause problems. The ECU read values from my 1998 shift lock control ECU with no issues.
You do NOT need to repin/replace your transmission. They are completely identical. I can switch between 1998 and 2001 ECUs with zero problems and zero transmission codes.

Sources:
User TW99GS300 information on cluster and ECU
User Meyagi with good information, but for Aristo gearboxes

See below for some wiring diagrams you may find helpful: